The present invention relates to branched polycarbonate compositions and a method for preparing them. More particularly, it relates to polycarbonate preforms comprising a branching agent and a method of making them via a melt transesterification process. The present invention further relates to a method for making blow moldable polycarbonate by melt equilibrating a mixture comprising a polycarbonate preform and a second polycarbonate. It also relates to the blow moldable polycarbonate which results from the method.
Polycarbonates are well-known high performance engineering thermoplastics characterized by many advantageous physical properties, including optical clarity, toughness, dimensional stability and excellent impact strength over a wide temperature range. To be suitable for use in blow molding applications, the polycarbonate resins must also exhibit sufficient melt strength and viscosity.
In the blow molding process, a tube of molten plastic is extruded from a die that is suspended above a mold. This tube, or parison, is then captured between the halves of the mold. Air is injected into the parison, expanding the plastic and forcing it against the walls of the mold thereby achieving the desired shape. For a resin to be useful in blow molding it must have sufficient melt strength and viscosity to hang from the die until it is captured between the two halves of the mold. It has been found that polycarbonate with a polyfunctional branching agent, such as 1,1,1-tris(4-hydroxyphenyl) ethane, intercondensed into the polymer backbone can provide the necessary melt strength and viscosity to ensure successful blow molding.
In the prior art, polyfunctional branching agents have been incorporated into polycarbonate that has been first polymerized by an interfacial method before introduction of the polyfunctional branching agent. Such a method is disclosed by Krabbenhoft et al. in U.S. Pat. No. 5,097,008 wherein a non-branched cyclic aromatic polycarbonate is contacted with a polyfunctional branching agent under melt polymerization conditions. Similarly, in U.S. Pat. No. 4,888,400, Krabbenhoft et al. contacted a non-branched linear aromatic polycarbonate with a polyfunctional branching agent under melt polymerization conditions. Polycarbonates that are useful in blow molding applications can be made by the two methods of these patents. However the level of branching agent which can be introduced into the polymer with this prior art method is typically below 2.0 mole percent to minimize the risk of gelation.
It would be desirable to produce a preform with a high loading of branching agent, preferably greater than 2.0 mole percent. Such a preform could then be melt equilibrated with any grade of polycarbonate to produce a blow moldable grade.
The prior art requires that branched polycarbonate suitable for blow molding be produced as a separate batch in a single step operation. This requires a manufacturing facility to interrupt production to change over to produce blow moldable grade. After completing the manufacturing run of blow moldable material, it is then necessary to again stop production and change back to produce standard grades.
It would be preferable for a manufacturing plant to continuously produce a standard grade of polycarbonate without interruption. This standard grade could subsequently be melt equilibrated in a separate operation with a preform to produce blow moldable polycarbonate. This would eliminate the need to break into a manufacturing schedule whenever blow moldable polycarbonate is required.
It would be convenient to introduce a multifunctional branching agent via coextrusion with a polycarbonate. However, addition of the multifunctional branching agent monomer to a polycarbonate resin can be accompanied by a reduction of molecular weight compared to the molecular weight of the starting resin. This resin weight reduction is observed with any hydroxy-containing additive or reagent under conditions where the additive will react with the polycarbonate backbone. Branched polycarbonates prepared by the Krabbenhoft methods can also exhibit a reduction in molecular weight and in viscosity after branching when compared to the non-branched starting material. It would also be desirable, therefore, to provide a method of incorporating branching agent into the polycarbonate without significantly reducing the molecular weight or the viscosity in comparison to that of the non-branched polycarbonate resin.